As will be appreciated by those skilled in the art, military applications of electro-optical systems require a knowledge of low-absorption coefficients of atmospheric constituents. This is so, for example, whether high energy lasers are utilized, or long-range laser and broadband systems, such as laser designators, range finders, image intensifiers, infrared images, and remote wind sensors, etc., are employed. Techniques for making these measurements have typically employed long-path absorption cells, but more recently, path differencing systems have been constructed which greatly reduce the data acquisition time and increase the measurement accuracy available. In essence, these advances effectively give to the massive cells and intricate optical and detection systems of the single-beam long-path absorption cell arrangements the real-time calibration of smaller double-beam spectrometer systems.
However, whereas the path differencing system of measurement represented a significant improvement over the long-path absorption cell arrangement, two persons were needed to perform the measurements--one, at the drive controls for the system, at one end of the cell, and the other, at the output of the system, the opposite end of the cell, some meters away. Additionally, some difficulty was experienced in rapidly and accurately repositioning and changing the beam pathlength, due, in part, to the limited torque capabilities of the available linear drives for the vacuum feedthrough pressure rods employed.